[Comm2011] Fwd: PPRS-041009-Updated-VST-CASSEGRAIN ROTATOR-rotator jumps when tracking-Electronic

[Konrad Kuijken]

Begin forwarded message:

> From: Paranal_AREmail <esoarspl at eso.org>
> Subject: PPRS-041009-Updated-VST-CASSEGRAIN ROTATOR-rotator jumps when tracking-Electronic
> Date: August 10, 2012 0:55:10 GMT+02:00
> To: kuijken at strw.leidenuniv.nl
> Reply-To: Paranal_AREmail <esoarspl at eso.org>
> 
> Problem number PPRS-041009 in the Paranal PRS has now been updated.
> The new details of the ticket are listed below.
> This is for your information only.
> 
> 
> Problem Number  : PPRS-041009
> 
> Creation Date   : 2011-08-03 01:39:12  by Thomas Szeifert  (vst at eso.org)
> Type            : Problem
> Keywords        : rotator jumps when tracking
> Priority        : Necessary
> Deadline        : 
> Status          : On Hold
> 
> -----  Minor Change Section ---------------------
> 
> -------------------------------------------------
> 
> Location             : VST
> Area/Instrument Type : TELESCOPE
> System/Instrument    : CASSEGRAIN ROTATOR
> Sub-system           : ROTATOR
> 
> Assigned to Group    : Electronic
> Assigned to Person   : rparra
> CC List              :
> mbernard at usm.lmu.de
> kuijken at strw.leidenuniv.nl 
> par-tios at eso.org 
> rtamai at eso.org 
> sguniat at eso.org 
> uweilenm at eso.org 
> msterzik at eso.org 
> vst at eso.org
> par-vst at eso.org 
> akaufer at eso.org 
> jspyromi at eso.orgVst_all 
> acarlier at eso.org
> 
> Telescope Downtime   : 00:00
> Workload (hrs.)      : 21
> Detailed Description :
> It was found that the rotator position jumps when tracking (yesterday as well when offsetting) by about 0.85 degree, which is a lot of a mechanical change/backlash. Could this be a problem with the encoders? Is it possible to check the encoders as soon as possible, since this is an obstacle for the OMEGACAM commissioning. VST team could not further investigate the issue before their departure.  Yesterday it jumped when tracking at -164 degree absolute angle.
> 
> On shorter time scales, before and after the jump, the tracking was very good.
> 
> -----  Worklog & Comments Section ---------------
> 
> Worklog and Comments : 
>   2011-08-03 01:43:27   vst
>      Yesterday night it was invetigated that the jumps were found when the direction changed, indicating backlash.
>   2011-08-04 23:42:17   vst
>      I was misunderstanding a comment from the team. It was all backlash related, no jump when tracking.
> 
>      Status Changed: On Hold 
>      Modified By: vst 
>      Modified Date: 2011-08-04 23:42:16 
>      Status Changed: On Hold 
>      Modified By: vst 
>      Modified Date: 2011-08-04 23:42:16 
>   2011-08-05 00:33:53   rparra
>      Tests were done in order to discard causes of the backlash:
> 
>      - Rotator initialized and then presetted to 0°. Then we put coincident marks at 0 deg. on fix and mobile part of the rotator in order to detect some backlash.
>      Test:
>      Preset -> -90° then Preset -> 0°  -> No backlash
>      Preset -> +90° then Preset -> 0°  -> appreciable backlash
>      Preset -> -90° then Preset -> 0°  -> no backlash, position 0° recovered.
>      On all the cases the position reported by the system was OK, showing zero degrees even while the marks were clearly missaligned.
> 
>      - Looking to encoders coupling at the rotator inside by means of a videoscope, no backlash was found.
> 
>      - Then, modifying the database we could disable the encoders and let just one encoder enabled. With only one encoder, the rotator doesn't show any backlash, even moving from -190° to 0°, or from +190° to 0°. This is true for any of the four encoders.
>      So, we can reasonably conclude that the reported backlash is not caused by mechanics nor by encoders.
> 
>      - Next test was to enable encoder pairs. Encoder couples accepted by the system are 1&3, and 2&4. On both cases the backlash is present, similar to the first tests done with all encoders enabled.
> 
>      - For testing purposes we left just one encoder (# 1) enabled for this night.
> 
>      Cristian, API, GVA, RPA,
>   2011-09-14 21:31:46   vst
>      We are back on 4 encoders. That is why the rotator was in a bad position last night. On ltvadc tracingAxis:vta:rot:POSLOOP:ABSENCODER deviceXUsed X = 2, 3, 4 were back to 1 now in the DB
>   2012-08-09 22:53:59   fsalgado
>      Assignee changed: rparra 
>      Modified By: fsalgado 
>      Modified Date: 2012-08-09 22:53:59 
>      Assignee changed: rparra 
>      Modified By: fsalgado 
>      Modified Date: 2012-08-09 22:53:59 
> 
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